Hi @exino and welcome to the community,
Our setup uses a similar idea. We have one email address and use mail rules to send them to seperate folder based on the project.
Within JIRA you can set up a mailhandler and when using an IMAP incoming mail server you can select a different folder than the INBOX.
So you'll have to move the messages based on their domain to a "project" folder in the mailbox which can then be read by individual mail handlers.
I don't think this can be done native.
I'm assuming by your request about domain (instead of username or group) that your servicedesks are open for unknown users?
You could look into an app called email this issue, where you can process incoming mails based on content, sender address and many more.
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